Prior art OFDM systems use either differential modulation or pilot symbol based coherent modulation. With differential modulation, there is little need for the transmission of known, or pilot, symbols. The use of differential modulation reduces overhead but does not allow for measurement of the channel response, also referred to as the complex channel gain, of the desired signal.
Pilot symbol based coherent modulation enables the channel response of the desired signal to be measured. With pilot symbol based coherent modulation, known pilot symbols are periodically transmitted along with the data symbols. The pilot symbols are typically spaced in time and frequency according to the expected rate of channel variation in time and frequency, respectively. The receiver compares a received pilot symbol to the known transmitted pilot symbol value at each pilot symbol location to measure the channel response at the pilot symbol locations within the time-frequency grid. Filtering is then used to interpolate values of the channel response between pilot symbols and reduce the noise on the estimates. This information is used by the receiver in the coherent data symbol detection process.
However, a limitation of both differential modulation and pilot symbol coherent modulation is that neither provide the capability to individually measure the channel responses of co-channel interfering signals. This limitation could be detrimental in communication systems which re-use frequencies, such as cellular radio communication systems which employ cellular frequency re-use patterns.